Polyorganophosphazenes are polymers having a backbone of alternating phosphorus and nitrogen atoms and for this reason have been referred to as "inorganic polymers." Each phosphorus, however, carries two substituent groups which can be inorganic, e.g. chlorine, or organic, e.g. phenoxy. The polymers may be low molecular weight such as cyclic trimers, tetramers and the like or oligomers containing about 3-20 repeating --P.dbd.N-- units. The more useful polymers are the high molecular weight open-chain polymers such as those described in U.S. Pat. Nos. 3,515,688; 3,702,833; 3,856,712 and 3,856,713. These polymers are soluble in solvents such as tetrahydrofuran (THF), cyclohexane toluene, etc. but can be cured (i.e., cross-linked) to insoluble polymers by means such as sulfur vulcanization or free-radical (e.g., peroxide) catalysts. Curing is more readily accomplished if the soluble polymer contains some olefinically unsaturated groups such as ortho-allylphenoxy (e.g., U.S. Pat. Nos. 3,970,533; 3,972,841 and 4,116,785).
Reynard et al. U.S. Pat. No. 3,948,820 describe open-chain polyorganophosphazenes having hydroxy substituents which can be crosslinked, by reaction with organic diisocyanates, e.g., toluene diisocyanate, to form a urethane-type cross-link. Reynard et al. U.S. Pat. No. 4,006,125 describes similar polymers wherein the hydroxy-substituted polyorganophosphazene is reacted with an excess of organic polyisocyanate such that the polymer is not cross-linked but is substituted through a urethane-type bond with an isocyanate terminated side chain. Such polymers can be cured by a small amount of water such as is present in moist air.